This invention relates in general to vehicle hydraulic brake systems and in particular to a brake system metering valve.
In "vertically" split vehicle hydraulic brake systems, a dual master cylinder is provided and includes separate front and rear brake hydraulic fluid reservoirs and actuating chambers. The actuating chambers are connected by hydraulic lines to the vehicle front and rear wheel brakes, defining separate front and rear brake circuits. Depressing a brake pedal connected by a mechanical linkage to the master cylinder applies hydraulic pressure to both brake circuits to apply the front and rear wheel brakes and slow the vehicle. Brake systems further include a normally open, or normally closed, brake light switch which is typically mounted adjacent to the brake pedal linkage. Movement of the linkage when the brake pedal is depressed actuates the brake light switch to illuminate the vehicle stop light lamps. The brake light switch is known to perform other functions, such as interrupting the vehicle cruise control.
During brake application, the weight of the vehicle shifts from the the rear wheels to the front wheels. This weight shift requires a greater braking force from the front wheel brakes than from the rear wheel brakes. Accordingly, vehicles typically have higher efficiency disk brakes for the front wheels and conventional drum brakes for rear wheels. Additionally, vehicles can include a proportioning valve in the rear brake circuit between the master cylinder and the rear wheel brakes. The proportioning valve functions to apply pressure to the rear wheel brakes at a lower pressure than is applied to the front wheel brakes.
The brake system also can include a metering valve inserted in the front brake circuit between the master cylinder and the front wheel brakes. The metering valve functions to delay application of the front wheel disk brakes until the hydraulic pressure has increased sufficiently to overcome the rear wheel drum brake return springs. Without a metering valve, the front disk brakes would be applied before the rear drum brakes, causing the front brakes to provide all the braking force during easy stops and thereby accelerating wear of the disk brake pads.
Brake systems having a dual master cylinders also typically include a differential pressure valve connected between the front and rear brake circuits to monitor the pressure therein. A difference in pressures sensed between the brake circuits is indicative of a component failure in the brake system and actuates a pressure differential switch to illuminate a brake failure warning light.
It is known to integrate the functions of the proportioning and metering valves and the pressure differential switch into one structure, commonly referred to as a combination valve. Such combination valves are adapted to be installed between the master cylinder and the wheel brakes.
Currently many vehicle brake systems also include anti-lock braking devices. Brake systems so equipped are referred to as anti-lock brake systems (ABS). Anti-lock brake systems typically include an ABS control valve which can be included in the brake system between the combination valve and the wheel brakes. An ABS computer control module controls the control valve to modulate the hydraulic pressure applied to the wheel brakes in response to signals generated by wheel speed sensors to prevent wheel lock-up during braking cycles. The ABS computer control module is actuated, or armed, when the brakes are applied.
It is known in the art to use the vehicle brake light switch to generate an arming signal for the ABS. In such a system, the brake light switch is electrically connected to the ABS control module. Actuation of the brake light switch generates a first electrical signal which is used to arm the ABS control module. Similarly, deactuation of the brake light switch upon release of the brake pedal generates a second electrical signal to disarm the ABS control module. Because the brake light switch is typically located remotely from the ABS control module, additional wiring is usually required to connect the brake light switch to the computer.
Typically, the ABS computer control module monitors the condition of the brake light switch. If the brake light switch remains actuated for longer than a predetermined time period, the control module decides that the brake light switch is defective and illuminates an ABS failure warning light. This is intened to preclude continual operation of the ABS if the brake light switch has failed.